This invention relates generally to gas turbine engines and, more particularly, to methods and apparatus for assembling rotatable machines.
Gas turbines are used in different operating environments, such as, to provide propulsion for aircraft and/or to produce power in both land-based and sea-borne power systems. At least some known gas turbine engines include a core engine having, in serial flow arrangement, a fan assembly and a high pressure compressor that compress airflow entering the engine. A combustor ignites a fuel-air mixture that is then channeled through a turbine nozzle assembly towards low pressure and high pressure turbines. The turbines each include a plurality of rotor blades that extract rotational energy from airflow exiting the combustor.
At least some known commercial jet engine fans operate with a relative blade tip Mach number in the transonic regime and may be subject to an operating characteristic known as multiple-pure-tone (MPT), or buzzsaw noise. Such noise may occur if at least some blades are oriented differently relative to other blades extending around the circumference of the fan case. Moreover, such noise may occur if blade-to-blade geometry variations exist within the fan and/or if flowfield disturbances are present upstream from the fan inlet. Such flowfield disturbances may be caused by any number of factors including, but not limited to drain leakage, panel splice leakage, and/or other geometric nonuniformities. As a result, variations may exist within the fan assembly in the amplitude (strength) and/or spacing of the shockwaves originating from those portions of the blades that have sonic or supersonic velocities. Specifically, at axial locations closer to the fan blades, the noise generated due to the shock waves is generally at multiples of the fan shaft per revolution frequency, which is the frequency with which one point on the shaft passes any particular fixed point as it rotates.
Shock waves of different strengths may propagate at different speeds. Accordingly, as a shock wave travels away from the blades, the noise at a blade passing frequency degenerates into a broad spectrum of lower frequency tones as the shock waves merge with each other. Buzzsaw noise may be an issue with passenger annoyance and comfort, and may also exceed preestablished community noise levels.
To facilitate minimizing multiple pure tone noise of the fan during operation, at least some known fan assemblies are assembled in a controlled manner. For example, at least some known fan assemblies are assembled such that each fan blade is mapped into a specific slot in the fan base. Within other known fan assemblies, a moment weight of each fan blade is determined and is used to map each blade into specific fan base slots. However, because some of the blades of a fan may have a geometry, moment weight, or other physical parameter that is so different from the remaining blades, resequencing the blades may not be capable of reducing fan MPT below acceptable limits.